Set gypsum, which comprises calcium sulfate dihydrate, is a well-known material that is included commonly in many types of products, such as gypsum boards employed in typical drywall construction of interior walls and ceilings of buildings. Typically, such gypsum-containing board is prepared by forming a mixture of calcined gypsum, that is, calcium sulfate hemihydrate and/or calcium sulfate anhydrite, and water, as well as other components, as desired. The mixture typically is cast into a pre-determined shape on the surface of a conveyor or in a tray. As it travels along the conveyor, the calcined gypsum reacts with water to form a matrix of crystalline hydrated gypsum or calcium sulfate dihydrate. The desired hydration of the calcined gypsum is what enables the formation of an interlocking matrix of set gypsum crystals, thereby imparting strength to the gypsum structure in the gypsum-containing product. Mild heating can be used to drive off unreacted water to yield a dry product. Gypsum mixers and methods of producing gypsum products are described, for example, in U.S. Pat. Nos: 1,767,791; 2,253,059; 2,346,999; 4,183,908, 5,683,635; 5,714,032; and 6,494,609.
Accelerator materials are commonly used in the production of gypsum products to enhance the efficiency of hydration and to control set time. Accelerators are described, for example in U.S. Pat. Nos: 3,573,947; 3,947,285; 4,054,461; and 6,409,825. Some accelerators include finely ground dry calcium sulfate dihydrate, commonly referred to as “gypsum seeds.” The gypsum seeds enhance nucleation of the set gypsum crystals, thereby increasing the crystallization rate thereof. Traditionally, accelerators have been added to the same mixer chamber as that used to combine water with calcined gypsum. While addition of accelerator to the mixer has the advantage of mixing the accelerator well and evenly throughout the water and calcined gypsum mixture, the accelerator can also cause the gypsum to begin setting prematurely. This premature setting causes the mixer to clog, can cause damage to the mixer, limits efficiency, and necessitates more frequent mixer cleaning. Mixer cleaning requires shutting down a board line with a serious detriment to productivity. Although additives including retarders have been used in the mixer to combat premature setting, such additives contribute additional costs and considerations.
Conventional gypsum seed accelerator materials progressively lose their effectiveness upon aging, even under normal conditions. In this respect, some efficiency of the accelerator is lost even as it is ground, and the gypsum seeds dramatically lose potency over time during handling or storage. The loss of acceleration efficiency of conventional accelerator materials is exacerbated when the accelerator is exposed to heat and/or moisture. To combat the loss of efficiency of the gypsum seeds over time, particularly under conditions of heat, it is customary to coat the calcium sulfate dihydrate accelerator material with any of a number of known coating agents, such as, for example, sugars, including sucrose, dextrose and the like, starch, boric acid, or long chained fatty carboxylic acids including salts thereof. Conventional heat resistant accelerator materials are both ground and provided in dry form inasmuch as accelerator loses efficiency upon contact with moisture, for example, because the accelerator particles undesirably agglomerate and/or because the coating agents often are soluble in water.
New materials and methods are needed to overcome the deficiencies of heat resistant accelerator that still preserve the benefits of using such an accelerator.